That is odd. When I searched for Vitamin D, it showed me a list (which I could sort by $/mg). That list included Vitamin D, Vitamin D2 and Vitamin D3. I assumed (incorrectly it seems) that this meant that it was doing some sort of prefix match. It seems that the result table highlights the "Ingredient" column, so I assumed (incorrectly) that it was searching that column. However, it doesn't seem to search the "Supplement" column, nor is it doing a prefix match (or substring match) on the Ingredient column. This is just confusing.
When you're typing things into search, you're using a freeform search which is going to match the ingredients based on the partial match.
If you want to look for the specific ingredients, you can do so by clicking on the "Ingredients" of the product example: Here are the links for the Vitamin D and D3. https://pillser.com/vitamins/vitamin-d and https://pillser.com/vitamins/vitamin-d3
There is even a Google Sheet that is generated based on the data that we have, although I just noticed that it's not updating properly, so I need to fix that.